This invention relates in general to a power supply and more specifically to a system for supplying power for arc welding and for plasma cutting and to an improved plasma cutting power supply.
Arc welding and plasma cutting are two processes which are frequently used together in many applications in the welding industry. A piece of metal may first be cut using a plasma cutting process and then welded to a different metal piece using an arc welding process. Both processes require power supplies which supply power of certain characteristics to welding torches or to cutting torches. In conventional welding systems, two separate power supplies have been used, one for performing plasma cutting and the other for performing arc welding. Plasma cutting requires a power source which provides power at high voltages and low current: for example, for cutting metal of up to one-half inch, the voltage and current supplied are typically at 100-120 volts and 20-50 amperes. On the other hand, arc welding requires a power source which can provide power at high current and low voltage, typically at 250 amperes and 20-30 volts for welding metal of up to one-half inch in thickness. Because of these different requirements for plasma cutting and arc welding, two separate power supplies have been used in conventional welding and cutting systems.
When two separate power supplies are used for cutting and for welding, there must be two separate electrical connections to power mains. The operators of the welding systems must keep track of and maintain two different power supply units instead of one. Two power supply units must be transported and stored on the shop floor. It is therefore desirable to provide a single power supply unit which is capable of providing power at high voltage and low current as well as at high current and low voltage suitable for plasma cutting and arc welding.
In U.S. Pat. No. 4,410,788, Summers et al. describe a power and fluid supply source with multifunction cutting and welding capabilities. The power supply includes a main power transformer for supplying the main welding current and a separate very high voltage transformer for supplying very high voltages above 200 volts required in conventional cutting systems for plasma cutting. The two transformers are placed in parallel and a switch is used to selectively connect one of the two transformers to the output. In other words, Summers et al.'s device includes the essential components of two different power supply sources, one suitable for arc welding and the other suitable for plasma cutting. Thus Summer et al.'s device merely includes in one unit the essential components of both sources and further adds a switch for selecting one of the power units to be effective at any one time.
While Summers et al.'s device enables welding operators to handle only one supply source instead of two, little economic advantage is achieved. Summers et al.'s device appears to be just as heavy as the combined weight of a conventional cutting and a conventional welding power supply and appears to occupy essentially the same floor space. In fact an extra coupling switch must be used for selecting between the main power transformer and the very high voltage transformer. Summers et al.'s power supply is thus not entirely satisfactory. It is therefore desirable to provide an improved power supply suitable for both plasma cutting and arc welding.
As discussed above, one of the challenges in developing a combined plasma cutting and arc welding power supply arises from the fact that plasma cutting requires a much higher voltage compared to arc welding. In plasma cutting, an inductance is frequently used in the power supply circuit for stabilizing the arc current so that the cutting arc is smooth and stable. This is well known to those skilled in the art. Upon arc iniatiation, such stabilizing inductance would resist the passing of current so that a high open circuit voltage in the range of 260 to 360 volts is normally required to overcome the stabilizing inductance so that the cutting arc would not become extinguished after initiation. Such high open circuit voltage is undesirable since it is a shock hazard to the operator of the cutting equipment. Such high voltage requirement also makes it more difficult to develop a combined plasma cutting and arc welding power supply. It is therefore desirable to provide a power supply which retains the normal circuit stabilizing inductance for a good smooth, stable cutting arc, and which also enables a much lower open circuit voltage to be used for sustaining the arc after arc initiation.